In the semi-conductor industry, large scale integration (LSI) and very large scale integration (VLSI) based systems require non-standard printed circuit fabrication techniques to accomodate the resulting high density, high current capacity printed circuit designs. Similarly, multi-layer microwave integrated circuits, large area flat panel displays and high resolution non-impact print heads require increased resolution and increased current-carrying capability. Moreover, as the complexity of integrated circuits increases, the number of input/output terminals increases. To accommodate this increasing number of input/output terminals, while at the same time limiting device package size, integrated circuits (ICs) are mounted in small ceramic chip carriers instead of being provided as conventional dual-in-line packages. Conductor patterns extend from the IC out to the exterior portion of the chip carrier where reflow soldering connects the chip carrier to the printed circuit board. In order to accommodate a typical chip carrier with as many as 64 input/output terminals, the printed circuit board must employ interconnected multiple layers of very densely packed conductors.
For applications such as these, neither conventional etched printed circuit techniques involving the so-called "subtractive process", nor hybrid thick film techniques have adequate pattern definition, the maximum definition with these processes being 80 lines per inch (5 mil linewidth). On the other hand, hybrid thin film processes which produce the requisite pattern definition of 150-400 lines per inch (4-1 mil linewidth) often cannot accommodate requirements for overall circuit board size and, more importantly, conductor thickness.
Thus, the metalizing of a conventional printed circuit board followed by etching away of preselected portions of the metalized layer in a "subtractive process", while providing conductors of sufficient thickness, does not provide for sufficient conductor definition to meet the high resolution requirements of high density packing.
Thin film processes, employing vapor deposition while providing for high resolution conductors, cannot provide for conductors of sufficient thickness to carry currents associated with the driving of electrostatic or electromagnetic print heads, high power transistors, silicon controlled rectifiers, triacs or other high power switching devices.
Additionally, with the increased usage of flat panel gas discharge displays there is an increasing need for relatively thick conductors. These conductors are used in fabricating a conductor matrix for applying power at selected crossover points to an ionizable gas between the conductors making up the crosspoint.
The problem in obtaining the requisite conductor thickness has been due primarily to the thicknesses at which photoresist material is obtainable. Liquid photoresists, in general, cannot be built up to a thickness of more than 0.5 mils, whereas dry resists, available in sheet form, are only available to a thickness of 2 mils. Since high current carrying applications often require conductors having thicknesses on the order of 4 to 6 mils, standard liquid and dry photoresists have not been able to meet thick film requirements. In order to achieve the requisite thick film conductors, it has oftentimes been necessary to utilize multiple metalization steps with attendant problems such as registration of a mask with an underlying substrate and/or patterned metal layer. The result of multiple metalization steps has been not only a lack of pattern definition but also a lack of homogeneity in the patterned structure itself.
One method of increasing pattern thickness is through the use of multiple resist layers. As discussed in a book entitled "Photoresist Material and Processes" by W. S. DeForest, published by McGraw Hill Book Company in 1975, multiple coatings have proved to be useful in electroforming applications that require thick plating. However, a bake-and-hold cycle with about ten minutes of baking at 180.degree. F. followed by a thirty minute cooling period is said to be necessary. Moreover, multiple exposures are said to be required with the first two resist layers being exposed prior to the lamination of a third layer. DeForest notes that after applying the third coating, the phototool must be realigned and normal exposure made. As stated by DeForest, alignment must be accurate. DeForest also states that developing will usually require two or more passes.
The utilization of multiple bake-and-hold cycles results in three highly deleterious effects. The first is resist shrinkage which results in delamination. Secondly, due to the normal mismatch of the coefficients of expansion between the resist and the substrate, the bake-and-hold steps may result in either delamination or stresses within the resist. Thirdly, all photoresists are, to some extent, heat sensitive. Multiple bake-and-hold cycles affect the photographic properties of the resist which has resulted in an inability to develop the resist. It should be noted that multiple exposures create alignment problems and multiple developing steps are costly and are apparently necessitated by the bake-and-hold cycles.
With respect to developing of dry photoresists, in general, dry photoresists are developed through the application of either a solvent or a mild alkaline solution usually applied by a coarse, high volume arrangement in which liquid streams of developer are directed over top of the exposed resist. Developing a resist in this manner, while satisfactory for most applications, does not result in a pattern having the highest degree of resolution possible. While airbrushing techniques, developed primarily at Lincoln Laboratories, have been useful in increasing the definition of certain thin film patterns, the utilization of an airbrush with thicker films tends to force the developer out of the crevices before the developer has a chance to operate.
By way of further background, as indicated in the DeForest book, dry film resists can be best categorized by the manner in which they are developed. DeForest thus categorizes dry resists in three categories, namely: Type I, the solvent-developing resists; Type II, the aqueous-developing resists; and Type III, the peel-apart resists.
Single layer resists as thick as 5 mils have been offered commercially in the past but have been removed from the market because of technical problems. As an example of the thickest type of aqueous-developing photoresist material currently available, a material designated Laminar AX dry film photopolymer is manufactured by the Dynachem Corporation, a subsidiary of Thiokol Corporation of Santa Anna, Calif. This resist is said to contain a mixture of polyfunctional acrylic or methacrylic monomers and a non-photosensitive binder resin. A mixture of unsaturated diallyl phthalate prepolymers is said to be used as a binder resin admixed with acrylic monomers such as pentaerythritol, diethylene glycol diacrylate and triethylene glycol diacrylate. The monomers are capable of forming highly interconnected three-dimensional network polymers upon exposure. Laminar AX is a blue, negative-working, dry photopolymer resist film supplied in thicknesses of 1.0, 1.5 and 2.0 mils which is sandwiched between a release sheet of 1 mil polyethylene and a cover sheet of 1 mil linear polyester. Thus, the maximum thickness that the Laminar AX dry film is available is two mils, a thickness which is insufficient to provide for the aforementioned thick film patterning of substrates.
It should be noted that U.S. Pat. No. 4,159,222 shows a method of manufacturing printed circuitry with a layer of resist having a flowable dielectric material laminated to the patterned structure. While this patent shows the use of multiple resists, the resists are not first laminated one to the other and are not patterned in one masking and exposure step.